New Insights into the Chemistry of the Hydrothermal Synthesis of Olivine Lithium Metal Phosphates

The effect of alkali ions (Na+ and K+) on the hydrothermal synthesis of olivine lithium metal phosphates in the P excess system was investigated by taking the synthesis of LiMn0.8Fe0.19Mg0.01PO4 as a case study. It is found that the coexisting Na+ can compete with Li+ to partly form NaMnPO4 as an impurity when the Na+/Li+ ratio exceeds the critical value, and this is not the case for the coexisting K+. These behaviors should be related to the difference in the ionic size of the alkali ions which also leads to a very different process of LiMn0.8Fe0.19Mg0.01PO4 formation. The samples derived from the K+-coexisting reaction system can deliver much higher reversible capacities as compared with the sample derived from the Na+-coexisting reaction system. This study provides new insights into the chemistry of the hydrothermal synthesis of olivine lithium metal phosphates, with an important implication for controllable synthesis and property manipulation.